window size of Go_back_N

Question

Two stations A and B are connected via a point to point link. A sends frames to B using Go-Back-N protocol with a window size of 7 and sequence number is of 3 bits. A sends all the frames one by one but B's ACK-3 for frame 3 is not reached to A. But before the timer expires, B received frame 4 and sent ACK-4 but frame 5 is lost and not received by B. So, now what will be frame buffer in the current window of A? ( assume frame number starts from 0)

a) 0 1 2 3 4 6

b) 3 4 5 6 0 1 2

c) 5 6 7 0 1 2 3

d) 5 6 0 1 2 3 4

Comments

Is it option C?

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Yes, it is. Can you please explain it?

Answer 1

GBN follows cumulative Ack. Hence it sends combined ack  for all before timer ends!

Here 3rd ack is not reached to A, but before timer ends 4th Ack is reached! Hence sender will think upto 4 all are reached! Now 5th packet is lost and sender will retransmit the whole frame from 5th packet!

seq numbers are 8 : 0-7 and frame size is 7

hence initial frame is

0 1 2 3 4 5 6

upto 4 all transmitted and 5th is lost

5 6 7 0 1 2 3 will be current window

Comments

point 1 – go back N can also use independent acknowledgement

point 2 – if it is using commulative ack. then why it send an independent ack. for 3 then a commulative ack. for 4?